Jumps

In Cairo each instruction takes a space of 1 or 2 field elements in memory.

pc is the Program Counter that keeps track of the current instruction's address and is advanced by 1 or 2 per instructions (depending on the instruction's size). The jmp instruction can be used to jump to different instructions.

There are 3 different jmp instructions one can use:

1. jmp abs: Absolute Jump
   • Jumps to a given address directly (e.g. jmp abs 42)
2. jmp rel: Relative Jump
   • Jumps to an offset relative to the current instruction (e.g. jmp rel 42)
3. jmp <label>: Label Jump
   • A Relative Jump where the Cairo compiler automatically resolves labels to their relative values (e.g. jmp foo)

Jumps can also be conditional which has the following syntax: jmp [<abs> | <rel> | <label>] if [<expression>] != 0. The <expression> has to be ap + offset or fp + offset (the offset is optional). Cairo will cause a jump to happen if the memory cell is not 0. You can write a 0 into the memory cell to not jump and continue with regular program execution.

Example

func main():
    # Write the value `10` to memory.
    [ap] = 10; ap++

    # This is our label we use in our `jmp` instruction below.
    countdown_loop:
    # We read the last value from memory, subtract `1` and write the new value to memory.
    [ap] = [ap - 1] - 1; ap++
    # We check if the last memory value is `0`.
    # If the value is not `0` we jump back to `countdown_loop`.
    # If the value is `0` we continue with our regular execution.
    jmp countdown_loop if [ap - 1] != 0

    assert [ap - 1] = 0

    ret
end